This invention relates generally to the field of molding apparatus for the encapsulation of objects in a plastic material. Specifically, the invention relates to a plate molding apparatus, of the type employing removable cavity plates, and advantageously used for the encapsulation of electronic components and circuits.
In the manufacture of semiconductor devices, it is the typical practice to provide a lead frame having a centrally-located support pad to which is attached a semiconductor micro-circuit or component, usually termed a "chip". Electrical connections are made between selected leads and appropriate terminals on the chip, and the chip, with a portion of the leads, is encapsulated in a suitable plastic encapsulation material.
It has become the common practice to provide the lead frames in continuous strip form, with a plurality of individual lead frames attached to one another to form the lead frame strips. After the chips are installed, encapsulation is performed by placing the strips in a mold having a cavity surrounding each chip, and the encapsulating material is then forced into the cavities to encapsulate the chips. After removal from the mold, the lead frame strips are separated, or "singulated", into individual encapsulated devices.
The molding apparatus used in the encapsulation process typically comprises upper and lower mold plates having complementary arrays of cavities. The lead frame strips are positioned on the lower mold plate with the portions to be encapsulated in registration with the lower mold cavities. The upper mold plate is then lowered onto the lower mold plate, with the cavities of the two plates in registration. One of the mold plates has channels or "runners" through which the plastic encapsulation material is injected. The runners communicate with the cavities through small branches or "gates", and the plastic thus flows through the gates to fill the cavities. When the plastic has hardened, the mold plates are separated and the lead frame strips with their encapsulated portions are removed by means of ejection pins provided in one or both of the mold plates.
While effective encapsulation is provided by the conventional molding apparatus described above, several disadvantages exist. For example, a thorough cleaning of the cavities, runners, and gates is required after each batch of strips is encapsulated. This results in a considerable amount of down time. Also, the mold plates are quite expensive, as they must be precision machined to relatively close tolerances, and the complex configuration of runners, gates, and cavities adds to their cost, and makes cleaning difficult. Furthermore, the ejection pins add a degree of mechanical complexity which contributes to the cost, while the pins themselves often mar the surface of the plastic encapsulation.
These problems have been recognized in the prior art, and solutions have been sought. One approach which has met with success is disclosed in U.S. Pat. No. 4,332,537 and U.S. Pat. No. 4,368,168, both to Slepcevic. The Slepcevic patents disclose a method and apparatus for encapsulating electrical components in an encapsulation mold having removable cavity plate means disposed between upper and lower mold plates. The cavity plate means has openings therethrough in which are positioned the objects to be encapsulated. Liquid plastic is forced through runners in the surface of the upper mold plate and then into the cavity plate openings through gates.
The plastic hardens in the cavities, which are in the shape of the encapsulation packages. After hardening, the plastic is ejected from the runners of the upper mold plate, and the cavity plate means is removed. Finally, the packaged, encapsulated objects are removed from the cavity plate means.
One characteristic of the Slepcevic method and apparatus is the formation of a seam or "parting line" around the middle of the molded package. With many types of packages, the existence of a parting line is of no consequence. For example, in the encapsulation of electronic components in dual in-line packages, or "DIP's" (which have leads extending from two opposed sides of the package), the existence of a parting line creates no particular problem, and is, therefore, acceptable.
However, with some types of packages, the existence of a parting line is not acceptable. This is particularly the case with single in-line packages, or "SIP's" (which have leads extending from only one side). Due to particular characteristics of the equipment used in handling and marking SIP's, the existence of a parting line can interfere with the operation of such equipment.
Thus, while the Slepcevic method and apparatus have found acceptance in the manufacture of DIP's, their use for SIP's has been impeded by the formation of the parting line. As a result, manufacture of SIP's has continued under conventional encapsulation methods, such as the chase molding method, with the disadvantages recited above.